Method of measuring the thickness of a slag layer on metal baths

ABSTRACT

The invention relates to a method for measuring the thickness of a slag layer on metal baths in which a probe connected to an electrical circuit and whose front surface is electrically conductive is lowered with constant speed through the slag layer into the metal bath, the electrical conductivities of the slag and the bath, or their electrical resistances respectively, being recorded in dependence on the speed with which the probe is lowered. Thus, when e.g. melts are to be degassed by vacuum treatment by means of a degassing vessel immersing with a tube into the slag-covered bath and which is periodically lifted and lowered, it is no longer necessary to maintain a great safety distance between the upper dead point and the metal surface. It is advantageous to provide for the upper dead point close to the metal surface because thus the lift height is increased which in turn affords a more rapid throughput of the metal.

United States Patent Jungwirth 1 51 May 16, 1972 54] METHOD OF MEASURING THE 3,396,960 8/1968 Maatsch ..75/60 x THICKNESS OF A SLAG LAYER 0 3,485,619 12/1969 Maatsch et al. ...75/60 3,505,062 4/1970 WOOdCOCk ..75/60 [72] Inventor: Adalbert Jungwirth, Linz Austria FOREIGN PATENTS 0R APPLICATIONS [73] Assignee; vel-einigte Osterreichische Eisemund 635,307 7/1963 Belgium ....75/6O Stah'werke Aktiengeseuschafl Linz, 800,177 8/1958 Great Br1ta1n ..75/49 tria Primary Examiner-Henry W. Tarring, ll Filed: P 6, 1970 AttorneyBrumbaugh. Graves, Donohue & Raymond [21] Appl. No.: 25,992 ABSTRACT The invention relates to a method for measuring the thickness Foreign Application pnomy Data of a slag layer on metal baths in which a probe connected to an Apr. 15, 1969 Austria ..A 3609/69 electrical circuit and whose front surface is electrically conductive is lowered with constant speed through the slag layer 52 U.S. c1 ..75/49,266/34.2, 75/60, into the metal bath, the electrical conductivities 9f the Slag 324/65, 73/304 and the bath, or their electrical resistances respectively, being 51 1111. C1. ..C21c 7/10, G01f23/26, GOlr 27/26 recorded in dependence Oh the Speed with Which the probe is 58 Field ofSearch. ..266/34 v, 34.2; 75/49, 60; icwctcd- Thus, when -12- mclts arc to bc degassed y vacuum 73/304; 324/ treatment by means of a degassing vessel immersing with a tube into the slag-covered bath and which is periodically lifted [56] References Cited and lowered, it is no longer necessary to maintain a great safety distance between the upper dead point and the metal UNITED STATES PATENTS surface. it is advantageous to provide for the upper dead point close to the metal surface because thus the lift height is in- 2,851,404 9/1958 Jackson BI al ..75/304 X creased which i turn affords a more rapid throughput of the 2,977,217 3/1961 Graef et al. ..75/60 metaL 3,062,994 11/1962 Mesh 75/304 UX 3,378,366 4/1968 Borowski et al. ..75/60 1 Claim, 2 Drawing Figures i 4 5 11 1 x x a 11 1 1 9 1 '1 #121 7 l 1 14 .i I

I ,11 1 1 1 5+ 11 1 1 17 t 111 l l 1 1 1 1 t 1 H 1 3 11 1 4 7: Gh'LnLn. 'LJ 5 if h n H 1 11 1 1 1 l t r 1 1i i i 1 l 6 ll .L..|

PATEMTEDMAY 16 I972 3,663 204 sum 2 or 2 INVENTOR. ADALBERT JUNGWIRTH M 226 M g M A T TORNE Y5 METHOD OF MEASURING THE THICKNESS OF A SLAG LAYER N METAL BATHS The invention relates to a method for measuring the thickness of a slag layer on metal baths. The establishment of the thickness of a slag layer in pig iron ladles, steel casting ladles, pig iron mixers and other metallurgical vessels is of great importance in metallurgy, on the one hand to afford the determination of the metal content of a vessel by ascertaining the exact position of the border surface between metal and slag and on the other hand for carrying out various metallurgical operations.

In steel degassing according to the siphon process the liquid steel to be treated is periodically sucked through the tube of a degassing vessel from a stationary ladle into said vessel to be degassed there whereupon it flows back into the ladle, said degassing vessel being capable of moving up and down. This means that the ladle content is degassed in portions, the individual portions being the greater, the greater the lift of the degassing vessel may be adjusted to be at a given ladle height. It is advantageous to provide for a quicker throughput of the steel content of the ladle because apart from saving time in the treatment, it is above all possible to reduce the invariable temperature loss which amounts to about 13 to 1.7 C. per lift. Thus it is attempted to fully make use of the possible lift height in order to shorten the time of treatment and it will be attempted to fix the upper dead point of the lifting movement as close as possible below the border surface between metal and slag. The upper dead point has to be chosen at a point where it is absolutely guaranteed that no liquid slag is sucked into the tube which would lead to the destruction of the refractory lining of the degassing vessel and would supply oxygen from the slag into the steel to be degassed which is highly undesired.

As up to now there was no way of determining the exact thickness of the slag layer which varies from ladle to ladle and it has been necessary to maintain a large safety distance between the upper dead point and the metal surface. Thus, the work was uneconomical.

The invention is aimed at avoiding the described disadvantages and difficulties and in a method described in the introduction, resides in that a probe connected to an electrical circuit and whose front surface is electrically conductive is lowered with constant speed through the slag layer into the metal bath, the electrical conductivities of the slag and the bath, or their electrical resistances respectively, being recorded in dependence on the speed with which the probe is lowered.

The method according to the invention thus utilizes the different electrical conductivities of slag and metal. At a temperature of e.g. about l,550 C. the electrical conductivity of the slags normally encountered in iron metallurgy thus amounts to about 0.2 Siemens, while the conductivity of the liquid iron amounts to 0.8 Siemens.

The method according to the invention is particularly useful for controlling degassing processes in which a degassing vessel immersing with a tube into a slag-covered metal bath is periodically lifted and lowered. In such method a measuring arrangement is used in which the measuring probe is fixed to the bottom of the degassing vessel and has a length that the electrically conductive front surface of the probe is situated at the same height as the mouth of the tube. By means of an appropriate switching, particularly a delay switching, it then becomes possible to interrupt the downward movement of the degassing vessel by a contact, such as a relay contact, as soon as the front surface of the probe reaches the metal bath surface.

In order that the method according to the invention may be more fully understood an embodiment thereof shall now be described with reference to the accompanying drawings, in which FIG. 1 serves to illustrate how the method according to the invention works in a siphon degassing process, and

FIG. 2 shows how the slag layer may be measured in any vessel.

In FIG. 1 numeral 1 denotes a steel ladle filled with liquid metal 2 which is covered by a slag layer 3. Above the ladle a degassing vessel 4 is arranged which is liftable and lowerable in the direction indicated by the double arrow 5. This degassing vessel is provided with a tube 6 projecting into the interior of the ladle. At the bottom of the degassing vessel a measuring probe 7 is releasably attached by means of a flange 8 which is screwed to the part 10 of the jacket of the degassing vessel by means of screws 9. An electrical insulation 11 is provided between the flange 8 and the part 10. The probe itself comprises an electrical conductor 12 and a refractory electrical insulating sheath 13. The length of the probe is such that its front surface is situated at the same height as the mouth of the tube 6. The probe is connected via the electrical conductor l4 fixed to a base 15, 16, with a current source 17. For the measurement direct current or alternating current with a voltage of 2-10 Volt is used. The circuit is formed from one pole of the current source to the electrode 12 and on the other side by a connection to the rails 18 on which a vehicle 19 stands with its wheels 20, which vehicle carries the refractorily lined ladle 1.

In FIG. 1 the front surface of the probe and thus the electrode just contacts the surface of the slag layer 3. When the probe 7 touches the slag a current flows through the continuously conductive ladle wall 21, the ladle vehicle 19, the wheels 20 and the rails 18, and via the recording instrument 22 and/or a relay coil 23 to the second pole of the current source 17. The recording instrument 22 comprises a recorder and a paper tape 25 which is moved in the direction indicated by the arrow 24. As soon as the measuring probe 7 touches the surface of the slag 3 the recorder will strike, indicating the intensity of current of the thus closed circuit. The current intensity thus increases from the value 0, indicated on the paper tape 25 by the broken line 26, to a value 27. When the electrode is lowered and touches the better conductive metal this current intensity changes to the value indicated schematically by the line 28. When the tape feed of the recording instrument is adjusted to run synchronous with the lowering movement of the electrode, the length of the line 27 will be equal to the thickness of the slag layer D. The length of the recorded line on the tape 25, which length corresponds to the depth of the slag layer, is illustrated by the borderlines 29, 30. The relay contact 23 interrupts the downward movement of the degassing vessel at the moment when the change in the conductivity between the two media occurs, i.e. when the probe reaches the metal bath surface. The probe heads position when it reaches the metal bath surface is denoted with 7'.

The tube 6 of the degassing vessel 4, which at its bottom end is provided with a slag fender 31, should be lowered below the metal bath level by a safety distance S before the degassing process starts.

When as has been described above the probe reaches the metal bath surface and the downward movement of the degassing vessel is stopped this movement is not immediately halted and a certain after running takes place. This fact may be considered when the length of the probe and the relative position of its bottom edge to the bottom edge of the tube is decided upon. In FIG. 1 the bottom edge of the measuring probe 7 and the bottom edge of the tube 6 are at the same level and the length of the after running corresponds to the desired safety distance S; thus, when the downward movement is stopped the tube will come to a standstill in the position 6 indicated by a broken line. When the downward movement is stopped the degassing process as well as the lifting and lower ing movement of the degassing vessel is started automatically, said latter movement commencing with the lift H. The positions of the tube and the measuring probe at the lower dead point are denoted with 6" and 7" respectively. The stroke movement is illustrated by the double arrow 5.

In FIG. 2 a ladle l is shown which is positioned on a ladle vehicle 19 exactly as in FIG. 1 and connected via the wheels 20 and the rails 18 by means of a similar switching arrangement as described in connection with FIG. 1, via the recording device 22 and the relay contact 23 with one pole of the current source 17. The ladle contains a metal bath 2 covered by a slag layer 3. The thickness of the slag layer is to be measured. A probe 7 formed exactly as described in FIG. 1 is suspended on a rope 32 which leads via electrically insulated rolls 33, 34

to the winch 35. The rope 32 is electrically conductive and the winch 35 is connected via the conductor 14 with the second pole of the current source 17. In order to measure the slag thickness the probe is vertically or, in the case of shortage of space, slantingly introduced into the ladle or into another metallurgical vessel with constant speed and the current throughout is recorded on the recording device 22 on a paper tape 25, The feed speed of the recorder tape and the speed with which the probe is lowered are adjusted relative to each other in a manner that the slag thickness may be read directly from the device. The weight 36 is suspended above the probe 7 on the rope 32 so that the probe may easily penetrate the slag layer and no mechanical resistance has to be overcome.

What I claim is:

1. A method for ascertaining the upper dead point in a degassing process of a melt by vacuum treatment wherein a degassing vessel having a tube projecting with its lower end into a ladle containing said melt covered by a slag layer is periodically lifted and lowered, comprising providing an electrical circuit including said ladle, a probe having an electrically conductive front surface and fixed to the degassing vessel in a manner that said electrically conductive front surface of said probe is situated at the same level as the lower end of the tube, and means for recording the electrical conductivities of said slag and said melt, lowering said degassing vessel together with said probe with constant speed through the slag layer into said melt, recording the electrical conductivities of said slag and said melt in dependence on the speed with which said probe is lowered into said melt, and interrupting lowering of the degassing vessel as soon as the probe reaches the melt, whereupon the degassing process is started. 

